Sandwich panels having cells formed in a honeycomb structure are well known. The cells of these known structures take on various shapes and may be evacuated. Examples of such known panels are found in U.S. Pat. Nos. 2,910,396; 2,944,504; 3,011,254; 3,030,703; 3,072,225; 3,137,602; 3,220,595; 3,664,906; 3,753,843; 3,895,152; 3,919,445 and 3,151,712.
Vacuum insulation involves the use of a double walled vessel having a high vacuum established within the space between the walls. This type of insulation substantially eliminates heat transfer by gaseous conduction and convection. Thus, when using vacuum insulation techniques, it is desirable to minimize heat transfer by radiation and conduction through solid structural members. Information concerning vacuum insulation found at pages 142 through 185 of the book entitled Cryogenic Engineering by Russell B. Scott published in 1959 by D. Van Nostrand Co. Inc. of New York, N.Y., is specifically incorporated herein by reference thereto.
The U.S. Pat. No. 3,501,367 discloses the use of a honeycomb core structure wherein the various plurality of cells are all interconnected and a vacuum pump is used to evacuate the entire honeycomb structure at the same time. A similar type of insulation product is shown in U.S. Pat. Nos. 2,837,779 and 3,990,201 both of which show the evacuation of a larger cell having individual sections defined by structure disposed therein.
The U.S. Pat. No. 3,993,811 discloses a plastic thermal insulating panel having a particular type of barrier material disposed on the surface of the plastic material to accomplish specific results in combination with gas absorbing materials such as activated charcoal. The vacuum that is used is disclosed to be down to about 10.sup.-2 mm Hg with a view to establishing a vacuum of about 10.sup.-1 mm Hg. The evacuation space is filled with a solid material.
The U.S. Pat. No. 4,167,598 discloses the use of elongated vacuum chamber panel elements which are contiguously disposed with respect to each other to form elongated panels. Thus, the elongated insulating cells are relatively large in size and there is no disclosure of the particular type of vacuum conditions which prevail in these chambers.
U.S. Pat. Nos. 3,137,602 and 3,365,897 disclose further honeycomb structures used for the construction of nose cones of missiles. The insulation panel of U.S. Pat. No. '897 is evacuated using cryo-vacuum techniques which involve the solidification of air and/or other gases in the cells at cryogenic temperatures.
It is well known that the thermal conductivity of air between atmospheric pressure and approximately 10 torr remains relatively constant. Then there is a sharp drop in the thermal conductivity as the pressure is decreased to about 10.sup.-3 torr. There is little discernable further decrease below 10.sup.-3 torr. At this level of vacuum, the heat conductivity through the evacuated volumetric space is substantially zero.
Energy transfer through a medium generally is due to conduction and radiation. The flow of heat through any surface is from a hot to a cold medium and is directly proportional to its area. Such flow of heat is directly proportional to the difference in the temperatures of the spaces separated by the surface. The amount of energy in Btu's flowing through one square foot of a surface medium per degree Fahrenheit in one hour is the conductivity of the material.
The amount of thermal energy transferred due to radiation is not dependent on a medium but is a function of light and reflectivity. Radiant heat can and does travel through a vacuum. The amount of solar radiated heat depends on the angle of the sun's rays, the color and roughness of the surface medium, the reflectivity of the surface and the type of construction used to form the surface. The intensity of radiant energy is directly proportional to the temperature of the source, and inversely proportional to the square of the distance, and travels at a speed of 186,000 miles per second in a straight line.